View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Open Research Exeter Ecological Entomology (2017), DOI: 10.1111/een.12448 Season length, body size, and social polymorphism: size clines but not saw tooth clines in sweat bees PAUL J. DAVISON∗ andJEREMY FIELD∗ School of Life Sciences, University of Sussex, Brighton, U.K. Abstract. 1. Annual insects are predicted to grow larger where the growing season is longer. However, transitions from one to two generations per year can occur when the season becomes sufficiently long, and are predicted to result in a sharp decrease inbody size because available development time is halved. The potential for resulting saw-tooth clines has been investigated only in solitary taxa with free-living larvae. 2. Size clines were investigated in two socially polymorphic sweat bees (Halictidae): transitions between solitary and social nesting occur along gradients of increasing season length, characterised by the absence or presence of workers and offspring that are individually mass provisioned by adults. How the body size changes with season length was examined, and whether transitions in social phenotype generate saw-tooth size clines. We measured Lasioglossum calceatum and Halictus rubicundus nest foundresses originating from more than 1000 km of latitude, encompassing the transition between social and solitary nesting. 3. Using satellite-collected temperature data to estimate season length, it was shown that both species were largest where the season was longest. Body size increased linearly with season length in L. calceatum and non-linearly in H. rubicundus but the existence of saw-tooth clines was not supported. 4. The present results suggest that because the amount of food consumed by offspring during development is determined by adults, environmental and social influences on the provisioning strategies of adult bees may be more important factors than available feeding time in determining offspring body size in socially polymorphic sweat bees. Key words. Body size, eusociality, size cline, social polymorphism, sweat bee. Introduction as Bergmann clines (BCs) (Bergmann, 1847; Ray, 1960, but see Watt et al., 2010), negative ones as converse-Bergmann clines Intraspecific geographic variation in life history traits is common (CBCs) (Park, 1949; Blanckenhorn & Demont, 2004). in many taxa (Roff, 1992; Stearns, 1992), and spatial variation in In seasonal environments, insects can grow and reproduce body size has received considerable research attention for more only during the active season (Bradshaw & Holzapfel, 2007; than 150 years (Blanckenhorn & Demont, 2004). In insects, body size can have a key influence on traits such as potential Gullan & Cranston, 2010), which becomes progressively shorter fecundity, resources allocated to offspring, thermoregulation, with increasing latitude and altitude (Bradshaw & Holzapfel, and overwintering success (May, 1979; Honek,ˇ 1993; Fox & 2007). Annual insects with long generation times can exhibit Czesak, 2000; Hunt & Simmons, 2000; Smith, 2002; O’Neill CBCs if a larger body size can be attained only by pro- et al., 2014). Within species, body size frequently varies either longing growth (Chown & Gaston, 1999; Blanckenhorn & positively or negatively with latitude and altitude (Chown & Demont, 2004). Correspondingly, CBCs are observed in insects Gaston, 2010; Shelomi, 2011). Positive relationships are known such as butterflies and crickets (e.g. Nylin & Svärd, 1991; Mousseau, 1997), and are frequently found to have a genetic Correspondence: Paul J. Davison, College of Life and Environ- basis (Masaki, 1967; Blanckenhorn & Fairbairn, 1995; Telfer mental Sciences, Centre for Ecology and Conservation, University & Hassall, 1999). In contrast, species with many generations of Exeter, Penryn Campus, Cornwall, TR10 9EZ, U.K. E-mail: per year in which growth is not limited by season length can [email protected] ∗Current address: College of Life and Environmental Sciences, Centre exhibit BCs, either because it is adaptive or as a consequence for Ecology and Conservation, University of Exeter, Penryn Campus, of temperature-mediated physiological processes (see Blanck- Cornwall, TR10 9EZ. enhorn & Demont, 2004 and references therein). © 2017 The Authors. Ecological Entomology published by John Wiley & Sons Ltd on behalf of Royal Entomological Society 1 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 2 Paul J. Davison and Jeremy Field Transition zone and body size Development time Development Univoltine Bivoltine Fig. 2. Being social takes longer than nesting solitarily. Brood rearing in the solitary life cycle is completed when offspring provisioned by the foundress emerge, but in the social life cycle offspring must provision a Season length second brood as workers. In both cases the life cycle must be completed before the end of the season. Fig. 1. A hypothetical saw-tooth cline, after Roff (1980) and Nygren et al. (2008). In socially polymorphic sweat bees, univoltine populations are solitary while bivoltine populations are social. lowland areas where the season is sufficiently long to rear two consecutive broods (Fig. 2; Soucy & Danforth, 2002; Davison & Field, 2016). Previous studies have generally found that sweat When the season becomes sufficiently long, fitness can be bees in more northern or upland areas are smaller and follow maximised by adopting a bivoltine life cycle (Masaki, 1972; CBCs (Richards & Packer, 1996; Soucy, 2002; Field et al., 2012; Roff, 1980), because any benefits of reaching a larger size Davison & Field, 2016, but see Sakagami & Munakata, 1972), are offset by increased mortality risk during development (e.g. although measurements have been made only at widely scattered Johansson & Stoks, 2005). As the time available for each sites hundreds of kilometres apart. Field et al. (2012) predicted generation to develop is halved at the transition from univoltine that socially polymorphic sweat bees could exhibit saw-tooth to bivoltine life cycles, Roff (1980) predicted that there should size clines, because double-brooded bees just to the south of the be a concomitant sharp drop in body size (Fig. 1). As predicted, transition might be more time stressed than single-brooded bees saw-tooth size clines are observed in a variety of groups where just to the north (Fig. 1). Body size is strongly correlated with there are latitudinal changes from univoltine to bivoltine life the amount and quality of food consumed during development cycles, including crickets (Masaki, 1972; Mousseau & Roff, (Plateaux-Quénu, 1983; Richards & Packer, 1994; Roulston & 1989), butterflies (Nygren et al., 2008), and moths (Välimäki Cane, 2002). Thus, offspring size might also be influenced by et al., 2013). However, saw-tooth size clines are not always environmental constraints on, and strategic investment decisions found and the relationships between body size and season by, adult bees at the time of provisioning (Richards & Packer, length may be complex (Kivelä et al., 2011; Välimäki et al., 1996; Field et al., 2012; Richards et al., 2015). 2013). For example, counter gradient variation occurs where In the present study, how transitions in social phenotype can a higher growth rate evolves to counter the effect of a shorter impact on the body size of foundresses is examined. The pres- available development time, which can over, under, or perfectly ence is tested for of saw-tooth clines in two socially polymorphic compensate for clinal variation in development time (Conover sweat bees along a gradient of increasing season length, from & Schultz, 1995; Blanckenhorn & Demont, 2004). the north of the United Kingdom (UK) southwards to western Socially polymorphic sweat bees (Hymenoptera: Halictidae) France. Lasioglossum calceatum Scopoli and Halictus rubicun- are a group in which the presence of saw-tooth clines might dus Christ are widely distributed throughout the Palaearctic and have more far-reaching implications. The same species can Holarctic, respectively, (Pesenko et al., 2000; Pesenko, 2005), exhibit both solitary and social behaviour, characterised by and each is socially polymorphic throughout its range (Sakagami either the absence or presence of a first brood of workers & Munakata, 1972; Soucy & Danforth, 2002; Field et al., 2010, before the production of reproductives (Fig. 2; Schwarz et al., 2012; Davison & Field, 2016). In the UK, both species nest 2007). Each spring, mated females (foundresses) emerge from socially in southern or lowland areas, but solitarily in northern hibernation and initiate subterranean nests. Foundresses then and upland environments (Field, 1996; Soro et al., 2010; Field mass provision a series of separate brood cells with a ball of et al., 2012; Davison & Field, 2016). The present results gener- pollen and nectar, providing each offspring with all the food ally support the existence of CBCs in both species, but not the required for development. In solitary nests, all female offspring existence of saw-tooth clines. mate and enter directly into hibernation. In social nests, however, B1 females are typically smaller than the foundress (Packer & Knerer, 1985; Schwarz et al., 2007), and at least some remain Materials and methods at the nest as workers to help rear a second brood (B2) of reproductive offspring. Sampling range Whether social or solitary